Ba 1.2-x Cs x M 1.2-x/2 Ti 6.8+x/2 O 16 (M = Ni, Zn) hollandites for the immobilisation of radiocaesium
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.43
Ba1.2-xCsxM1.2-x/2Ti6.8+x/2O16 (M = Ni, Zn) hollandites for the immobilisation of radiocaesium D. J. Bailey1,*, M. C. Stennett1 & N. C. Hyatt1 1- Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, United Kingdom
ABSTRACT
Improved budgeting of heat loads arising from radiogenic heating in high level wastes (HLW) could allow enhanced usage of geological disposal facility space. Separation of high heat generating nuclides from HLW, such as Cs, would simplify management of heat loads. A potential host matrix for Cs-disposal is hollandite. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the structure; these ions may include Ni and Zn. Two series of hollandites, Ni-substituted and Zn-substituted, were synthesised via an alkoxide-nitrate route and consolidated by cold uniaxial pressing and sintering or by hot isostatic pressing. Characterisation of the resultant material by X-ray diffraction and scanning electron microscopy found that hollandite was formed for all levels of substitution. Materials produced via HIP were found to be denser indicating lower Cs loss. HIPed Ni hollandites were found to contain fewer secondary phases and it was concluded that they were the most suitable candidates
INTRODUCTION Cs is a major contributor to radiogenic heating in high level wastes (HLW). Potential separation of Cs and other heat generating radionuclides and immobilisation in separate wasteforms could greatly simplify the management of heat loading in a future geological disposal facility. A potential host for separated caesium is hollandite, A xByC8-yO16, where x ≤ 2 1. A site cations are large and either monovalent or divalent (e.g. Cs+, Sr2+, Ba2+, Ra2+, Pb2+). B and C site cations are more varied with the accommodation of di, tri, tetra, and pentavalent cations possible e.g. Ni2+, Fe3+, Ti4+, Sb5+ 1,2. The structure comprises corner and edge sharing BO6 and CO6 octahedra forming tunnels along the c-axis of the structure, A site ions are located within these tunnels and are typically eight-fold coordinated by oxygen preventing free migration along the tunnels 3. The radius ratio of
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A and B site cations dictates the crystal symmetry of the system and when R A/RB > 2.08 tetragonal symmetry results (I4/m). For systems where R A/RB < 2.08, the unit cell distorts resulting in the reduction of symmetry to a monoclinic system (C2/m, although this sometimes written as I2/m) 4,5. Barium titanate hollandites are the basis for many wasteform formulations, including the Cs-bearing component of SYNROC formulations 6,7, and have previously been synthesised using a range of additional B site cations including F
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